This invention relates generally to optical spectral monitors and analyzers. More specifically, it relates to a new class of optical spectral power monitors employing novel frequency-division-multiplexing detection schemes, which are well suited for WDM optical networking applications.
As optical communication networks employing wavelength division multiplexing (WDM) become increasingly pervasive, a new line of optical performance monitors, including spectral power monitors, is in demand.
Conventional optical spectral power monitors typically use a wavelength-dispersing means, such as a diffraction grating or a dispersing prism, to separate a multi-wavelength optical signal into a spatial array of spectral channels with distinct center wavelengths. An array of photo-detectors (e.g., photodiodes) is positioned to detect the spectral channels individually, thereby providing an optical power spectrum of the multi-wavelength optical signal. Alternatively, a rotating diffraction grating and a stationary photo-detector, or a movable photo-detector and a stationary diffraction grating, are used to scan the spectral channels sequentially. These prior spectral power monitors are typically high in cost, cumbersome in size and operation, and in some instances require considerable maintenance, rendering them unsuble for optical networking applications.
U.S. Pat. No. 5,631,735 of Nagai discloses a spectrometer that uses a diffraction grating for separating an incident light signal into respective wavelength components, impinging onto an array of optical shutter elements. The respective wavelength components are modulated with different frequencies, and subsequently multiplexed to one beam to be detected by a detector. The output signal of the detector is then demodulated in order to derive the intensity of the respective wavelength components.
It is known in the art that the diffraction efficiency of a diffraction grating is characteristically polarization sensitive, and may also be wavelength dependent. However, no effort is made in the aforementioned spectrometer of Nagai to mitigate such effects. Moreover, there may be other systematic effects in a practical system, inflicting additional optical loss to an optical beam. As such, the intensity measurement provided by Nagai""s spectrometer is not reflective of the actual optical power level of the respective wavelength components in the input light signal.
U.S. Pat. No. 6,239,889 of Harley et al. discloses a scheme for optical signal power detection in a WDM system. A unique signature bit pattern is inserted in a digital optical signal, where the power level of the signature bit pattern is adjusted at the launching point to a predetermined ratio with the power level of the optical signal to be detected. At a point of interest, a fraction of the modulated optical signal is tapped off, and subsequently converted into an electrical signal. By extracting the fraction of the signature bit pattern from the electrical signal and measuring its power level, the power level of the optical signal can be determined. Such a scheme can also be extended to a plurality of optical signals multiplexed on an optical fiber, where each optical signal is given a unique signature pattern
The aforementioned method of Harley et al. is limited in its application, in that the signature bit patterns have to be inserted on a network level, before any power detection can be made at the point of interest. Moreover, it requires that the predetermined ratio of the power level of the signature bit pattern to the power level of the optical signal be maintained as the optical signal propagates in the network, which might be a difficult proposition in practice.
Therefore, it would be an advance in the art to overcome the shortcomings of the foregoing devices and methods and to provide a new line of optical spectral power monitors that can be deployed anywhere in a WDM network and provide accurate detection of the WDM signals.
The present invention provides a method and apparatus for optical spectral power monitoring by way of a novel frequency-division-multiplexing detection scheme. The optical spectral power monitoring apparatus of the present invention comprises an input port for a multi-wavelength optical signal and an output port; a wavelength-disperser that separates the multi-wavelength optical signal by wavelength into multiple spectral channels having a predetermined relative arrangement; and an array of beam-modulating elements positioned such that each beam-modulating element receives a unique one of the spectral channels. The beam-modulating elements are individually controllable such that the optical power levels of the spectral channels coupled into the output port carry distinct dither modulation signals.
In the present invention, a xe2x80x9cspectral channelxe2x80x9d is characterized by a distinct center wavelength and associated bandwidth, and may carry a unique information signal as in WDM optical networking applications. A xe2x80x9cdither modulation signalxe2x80x9d refers to any modulation in the optical power level of a spectral channel that is caused by the corresponding beam-modulating element, in contrast with other xe2x80x9cintrinsicxe2x80x9d modulation signals (e.g., information signals) the input multi-wavelength optical signal may carry. Accordingly, the dither modulation signals are allocated in a spectral range that is sufficiently separated from the frequencies of other xe2x80x9cintrinsicxe2x80x9d modulation signals the spectral channels may carry.
In one embodiment of an optical spectral power monitoring apparatus of the present invention, an array of micromirrors serves as the beam-modulating elements; the output port acts as a spatial filter, such that misalignment in coupling a spectral channel to the output port effectively causes the optical power level of the spectral channel coupled into the output port to change. The micromirrors may be under control of a set of mirror-control (e.g., voltage) signals, such that at nominal positions the micromirrors reflect the corresponding spectral channels into the output port according to predetermined (e.g., maximum) coupling efficiencies. The micromirrors may be further pivoted about the respective nominal positions by way of alternating (or xe2x80x9cditherxe2x80x9d) components in the corresponding mirror-control signals, whereby the optical power levels of the spectral channels coupled into the output port undergo distinct oscillations, hence the dither modulation signals. As such, the micromirrors thus described, along with the output port, effectively function as xe2x80x9cspatial light modulators.xe2x80x9d The micromirrors may be provided by silicon micromachined mirrors that are each pivotable about at least one axis, for example. The output port may be a fiber collimator, an aperture, or other spatial filtering means known in the art.
Furthermore, an optical detector may be optically coupled to the output port, so as to convert the received optical power levels to a consolidated electrical signal, which carries the same characteristic dither modulation signals. A synchronous detection unit, in communication with the optical detector, may be used to detect the individual dither modulation signals in the consolidated electrical signal. The characteristics of the dither modulation signals thus detected, along with a predetermined calibration table that takes into account any systematic effect on a channel-by-channel basis, provide an optical power spectrum (i.e., optical power level as a function of wavelength) of the input multi-wavelength optical signal.
The aforementioned embodiment may further employ a polarization diversity scheme, so as to mitigate any polarization-dependent effect the constituent optical elements may possess. In this case, a polarization-separating element and a polarization-rotating element may be disposed along the optical path between the input port and the wavelength-disperser, serving to decompose the input multi-wavelength optical signal into first and second polarization components and subsequently rotating the polarization of the second polarization component by 90-degrees. The wavelength-disperser separates the incident optical signals by wavelength into first and second sets of optical beams, respectively. A beam-focuser (e.g., a focusing lens) may focus the first and second sets of optical beams into corresponding focused spots, impinging onto the micromirrors. Upon being reflected by the respective micromirrors and further multiplexed by the wavelength disperser, the reflected first and second sets of optical beams may be re-combined by way of a polarization-combining element in conjunction with an auxiliary polarization-rotating element, prior to being coupled into the output port.
The reflected first and second sets of optical beams may alternatively be coupled into the output port and an auxiliary output port, whereby the optical power levels coupled into each output port carry distinct dither modulation signals. First and second optical detectors may be optically coupled to the output port and the auxiliary output port, respectively. First and second synchronous detection units may be in communication with the first and second optical detectors and serve to detect the dither modulation signals associated with the first and second sets of optical beams, respectively. The characteristics of the dither modulation signals thus detected, combined with predetermined calibration tables, enable an optical power spectrum associated with each polarization component in the input multi-wavelength optical signal to be separately determined.
In another embodiment of the present invention, the beam-modulating elements are provided by an array of spatial light modulators known in the art, such as electro-optic intensity modulators, in a polarization diversity scheme. In this case, the first and second sets of optical beams dispersed by the wavelength-disperser impinge onto the corresponding electro-optic modulating elements, followed by a beam-reflector (e.g., a mirror). The electro-optic modulating elements may operate under control of a set of distinct alternating (or dither) control signals, so as to introduce dither modulation signals in the optical power levels of the optical beams. As such, upon a round-trip through the respective electro-optic modulating elements by way of the beam-reflector and further multiplexed by the wavelength-disperser, the first and second sets of optical beams may be directed into first and second optical detectors, respectively, whereby the optical power levels received by each optical detector carry distinct dither modulation signals. First and second synchronous detection units may be in communication with the first and second optical detectors and serve to detect the dither modulation signals associated with the first and second sets of optical beams, respectively. The characteristics of the dither modulation signals thus detected, combined with predetermined calibration tables, enable an optical power spectrum associated with each polarization component in the input multi-wavelength optical signal to be separately determined.
In the present invention, the wavelength-disperser may generally be a ruled diffraction grating, a holographic grating, an echelle grating, a curved diffraction grating, a transmission grating, a dispersing prism, or other types of wavelength-separating means known in the art. The input port may be a fiber collimator. The polarization-separating element may be a polarizing beam splitter, a birefringent beam displacer, or other types of polarization separating means known in the art. The polarization-rotating element may be a half-wave plate, a Faraday rotator, a liquid crystal rotator, or any other polarization-rotating means known in the art that is capable of rotating the polarization of an optical beam by 90-degrees. The polarization-combining element may be a birefringent element, e.g., a birefringent beam displacer operated in reverse. It may also be other means known in the art that is capable of combining two optical beams by way of polarization.
The optical spectral power monitoring apparatus of the present invention may further include one or more reference signals and corresponding reference-position-sensing elements, along with one or more appropriate alignment-adjusting elements, for monitoring the alignment between the spectral channels and the respective beam-modulating elements and correcting for shift in the alignment that may arise over the course of operation.
As such, the present invention provides an optical apparatus that is capable of introducing distinct dither modulation signals in multiple spectral channels in a frequency-division-multiplexed fashion. The distinct dither modulation signals enable the multiplexed spectral channels to be individually detected (e.g., by way of synchronous detection), whereby an optical power spectrum of the spectral channels can be derived. Such dither modulation signals may also be used as xe2x80x9cidentification markersxe2x80x9d (or frequency tags) for identifying individual spectral channels in an optical networking application.
The novel features of this invention, as well as the invention itself, will be best understood from the following drawings and detailed description.